1. Field of the Invention
This invention relates to a method for the production of acrylic acid, for example, which method comprises preparing the chilled coolant by utilizing the latent heat generated during the gasification of propylene and/or propane as the raw material for the production of acrylic acid and circulating the chilled coolant to the heat exchangers being used in the apparatus for the production of acrylic acid, and to the apparatus for the production of acrylic acid which is suitable for the method mentioned above.
2. Description of Related Art
Acrylic esters have been finding utility as materials for acrylic fiber copolymers, as emulsions for tackifiers and adhesives, and as raw materials for paints, processed fibers, leather products, and buildings. They have been consequently enjoying growing demands. In the circumstance, the desirability of developing a process which allows use of inexpensive raw materials, permits mass production, and restrains any addition to the environmental pollution has been finding popular recognition. The acrylic acid and the like which are used as raw materials for such acrylic esters are now produced by the reaction of catalytic gas phase oxidation of propylene, for example.
An example of the process for producing acrylic acid by this reaction of catalytic gas phase oxidation will be explained with the aid of FIG. 1 depicting the flow of the raw material for acrylic acid with thick lines and the flow of cooling water with thin lines.
First, the reaction of catalytic gas phase oxidation requires supply of the raw material therefore in the form of the gas as aptly implied by the name. A liquefied propylene 14 is gasified through the propylene evaporator 3, gaseous propylene from the evaporator is mixed with air 16 for use in the reaction of oxidation, and the resultant mixture is subsequently supplied to the reactor 4. When propylene, for example, is subjected to catalytic gas phase oxidation with the molecular oxygen-containing gas in the presence of the catalyst for the reaction of catalytic gas phase oxidation, the mixed gas containing such by-products as acetic acid in addition to acrylic acid, i.e. the product aimed at is obtained as the reaction product. The reaction product gas formed by the reaction of catalytic gas phase oxidation, therefore, is introduced into the acrylic acid absorbing column 5 and exposed therein to the acrylic acid absorbing solvent, thereby cooled and absorbed, and eventually the liquid containing acrylic acid and such by-products as acetic acid as well is obtained. Then, the solvent in this liquid is separated in the solvent separating column 6 and subsequently distilled and purified in the refining column 7 to separate acrylic acid 18 from by-product containing liquid and obtain the product aimed at.
In this case, a part of the bottom liquid in the acrylic acid absorbing column 5 is introduced into the circulation cooler 9 attached to the acrylic acid absorbing column and cooled by exchanging heat with the liquid coolant supplied from a part of liquid coolant supplying system 1 and then circulated to the acrylic acid absorbing column 5. Incidentally, the acrylic acid absorbing column 5 is generally installed with the absorbing solvent cooler 8 adapted to cool the acrylic acid absorbing solvent 21 for the purpose of enhancing the efficiency of acrylic acid absorption. The distillate from the top of the solvent separating column 6 is condensed in the condenser 10 attached to the solvent separating column 6 with the liquid coolant supplied from a part of the liquid coolant supplying system 1, thereby recovering the solvent 22. Similarly, the distillate from the top of the refining column 7 is condensed in the condenser 11 attached to the refining column 7 with the liquid coolant supplied from a part of the liquid coolant supplying system 1, thereby obtaining the acrylic acid 18. The liquid coolant resulting from heat exchange can be supplied to the methacrylic acid and/or (meth)acrylic esters plant 12, subjected to heat exchange therein, and then put to use. In any event, the liquid coolant lines are provided for the purpose of allowing circulation of this liquid coolant from these heat exchangers to the liquid coolant supplying system 1. Incidentally, the acrylic acid absorbing column 5 discharges the waste gas 20 through the top thereof.
It has been heretofore customary to supply steam 17 controlled by the pressure controller 24 to the evaporator 3 for the purpose of utilizing the high energy of the steam 17, thereby gasifying liquefied propylene. The condensed drain of the steam 17 resulting from the heat exchange has been utilized as the boiler feed water for the purpose of harnessing the sensible heat thereof.
This method, however, is at a disadvantage in suffering even a slight fluctuation of the steam pressure or the amount of supplied steam 17 to render the vapor pressure and the dryness of the propylene gas unstable because the energy of steam is high. In the production of acrylic acid by the reaction of catalytic gas phase oxidation, therefore, the composition of the reactant gas tends to vary and the stability of the reaction system itself tends to lose stability. Further, the concentration of the acrylic acid in the gas supplied to the acrylic acid absorbing column 5 likewise varies and, as a result, polymerization and clogging will happen at the column inside, and the efficiency of absorption will drop in the consequence of the formation of polymer. The decline of the efficiency of absorption lowers the concentration of the acrylic acid in the liquid in which acrylic acid is absorbed emanating from the acrylic acid absorbing column, with the result that the load on the column will rise during the separation of the solvent by the solvent separating column 6.
In the production of the acrylic acid, the acrylic acid absorbing column 5, the solvent separating column 6, the refining column 7, etc. are provided for the purpose of purifying the acrylic acid obtained by the reaction of catalytic gas phase oxidation and these columns are each provided with numerous heat exchangers adapted to enhance the efficiencies of absorption, separation, and purification by utilizing the principle of heat exchange. As concrete examples of the heat exchanger of this nature, the acrylic acid absorbing solvent cooler 8, the condenser 11 attached to the refining column 7, and the condenser 10 attached to the solvent separating column 6 may be installed. The cooling water for use in these heat exchangers is supplied from the liquid coolant supplying system 1 such as a refrigerator and/or cooling tower. Since the temperature of the cooling water varies due to the season, there are cases where the capacity for heat exchange will drop as the rise of cooling water temperature. To prevent this temperature from rising, another electric power is required for cooling the liquid coolant.
Further, in the case of the solvent separating column 6, for example, the distillate from the top of this column is condensed and cooled by the condenser 10 attached to the solvent separating column. If the efficiency of absorption in the acrylic acid absorbing column 5 falls down, the load exerted to bear on the subsequent solvent separating column 6 will be possibly elevated and the load on the condenser 10 will be consequently elevated. Owing to this rise of load coupled with the fall of the capacity for heat exchange due to the rise of the temperature of the cooling water mentioned above, the distillate from the top of the solvent separating column 6 will not be condensed or cooled sufficiently. Further, the distillate contains acrylic acid, polymerizable by-products, etc. besides the solvent. The solvent separating column 6 is usually operated under vacuum pressure. Under these complex circumstances, insufficient condensation of the distillate will possibly induce such problems as increasing the amount of the polymerizable materials to be scattered in the vacuum equipment connected to the condenser, and cause the vacuum equipment and the piping installed thereto to incur such adverse situation as polymerization and clogging. As a result, it possibly becomes difficult or even impossible to control the operating pressure of the column. Further, these detriments possibly induce the polymerization in the solvent separating column and give rise to such troubles as deterioration of final product quality.
Heretofore, the problem of the polymer formation due to the supply of the steam 17 thereto has been coped with only by periodic removal of the polymer deposited thereon.